Biostimulator circuit with flying cell

ABSTRACT

A leadless cardiac pacemaker is provided which can include any number of features. In one embodiment, the pacemaker can include a tip electrode, pacing electronics disposed on a p-type substrate in an electronics housing, the pacing electronics being electrically connected to the tip electrode, an energy source disposed in a cell housing, the energy source comprising a negative terminal electrically connected to the cell housing and a positive terminal electrically connected to the pacing electronics, wherein the pacing electronics are configured to drive the tip electrode negative with respect to the cell housing during a stimulation pulse. The pacemaker advantageously allows p-type pacing electronics to drive a tip electrode negative with respect to the can electrode when the can electrode is directly connected to a negative terminal of the cell. Methods of use are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 61/678,505, filed on Aug. 1, 2012, titled “BiostimulatorCircuit with Flying Cell”, the contents of which are incorporated byreference herein.

INCORPORATION BY REFERENCE

All publications and patent applications mentioned in this specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

FIELD

This disclosure relates generally to implantable pacemakers orbiostimulators. More specifically, this disclosure relates to improvedimplantable leadless pacemakers having a reduced weight and volume.

BACKGROUND

Cardiac pacing electrically stimulates the heart when the heart'snatural pacemaker and/or conduction system fails to provide synchronizedatrial and ventricular contractions at appropriate rates and intervalsfor a patient's needs. Such bradycardia pacing provides relief fromsymptoms and even life support for hundreds of thousands of patients.Cardiac pacing may also give electrical overdrive stimulation intendedto suppress or convert tachyarrhythmias, again supplying relief fromsymptoms and preventing or terminating arrhythmias that could lead tosudden cardiac death.

Pacemakers require at least two electrodes to deliver electrical therapyto the heart and to sense the intracardiac electrogram. Traditionally,pacemaker systems are comprised of an implantable pulse generator andlead system. The pulse generators are implanted under the skin andconnected to a lead system that is implanted inside the heart with atleast one electrode touching the endocardium. The lead system can alsobe implanted on the epicardial surface of the heart.

Pacemaker lead systems are typically built using a unipolar design, withan electrode at the tip of the lead wire, or bipolar design, with anadditional electrode ring often 10 mm proximal to the tip electrode.Additionally, the implanted pulse generator can is often used as apace/sense electrode. In a conventional pacemaker system, pacing occurseither between the electrode tip and ring, or between the tip and can.Likewise, sensing occurs either between the electrode tip and ring orbetween the tip and the can.

SUMMARY OF THE DISCLOSURE

A leadless cardiac pacemaker, comprising an electronics housing, pacingelectronics disposed in the electronics housing, a tip electrodeelectrically coupled to the pacing electronics, a cell housing, and anenergy source disposed in the cell housing, the energy source having apositive terminal electrically coupled to the pacing electronics, and anegative terminal electrically coupled to the cell housing, the pacingelectronics being configured to drive the tip electrode negative withrespect to the cell housing during a stimulation pulse.

In some embodiments, electrically coupling the negative terminal to thecell housing configures the cell housing to act as a can electrode.

In one embodiment, the pacing and sensing electronics comprise at leastone p-type substrate.

In additional embodiments, the energy source comprises at least onelithium carbon mono-fluoride cell.

In some embodiments, the pacemaker does not include an additionalhousing or ring electrode disposed around the cell housing.

In one embodiment, the pacemaker is configured to provide stimulationpulses from the cell housing to the tip electrode through cardiactissue.

In some embodiments, the pacing electronics permit the cell housingwhich is coupled to the negative terminal of the energy source to serveas a positive can electrode during the stimulation pulse.

In another embodiment, the pacing electronics include at least oneswitch that prevent the passage of current in the presence ofdefibrillation or electrosurgery voltages on a high terminal of the atleast one switch.

A method of driving a leadless pacemaker is also provided, comprisingthe steps of coupling a negative terminal of a cell to a cell housing ofthe leadless pacemaker, coupling a positive terminal of the cell top-type substrate pacing electronics of the leadless pacemaker, driving,with the pacing electronics, a tip electrode of the leadless pacemakernegative with respect to the cell housing during a stimulation pulse.

In one embodiment, the method further comprises the step of stimulatingcardiac tissue with the stimulation pulse.

In some embodiments, the driving step comprises driving the tipelectrode as a negative electrode and driving the cell housing as apositive electrode during the stimulation pulse.

A leadless cardiac pacemaker is also provided, comprising a tipelectrode, pacing electronics disposed on a p-type substrate in anelectronics housing, the pacing electronics being electrically connectedto the tip electrode, and an energy source disposed in a cell housing,the energy source comprising a negative terminal electrically connectedto the cell housing and a positive terminal electrically connected tothe pacing electronics, the pacing electronics being configured to drivethe tip electrode as a negative electrode and the cell housing as apositive electrode during a stimulation pulse.

In some embodiments, the energy source comprises at least one lithiumcarbon mono-fluoride cell.

In another embodiment, the pacemaker further comprises a fixationfeature configured to affix the pacemaker to cardiac tissue.

In one embodiment, there is no separate housing disposed around the cellhousing.

In another embodiment, the cell housing is configured to act as a canelectrode.

In yet another embodiment, there is no separate ring or can electrodedisposed around the cell housing.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe claims that follow. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings of which:

FIG. 1 shows an external view of a biostimulator or leadless pacemaker.

FIGS. 2A-2B provide schematic diagrams of pacing electronics accordingto one embodiment.

FIGS. 3 and 4 provide detailed diagrams of the implementation ofswitches in the pacing electronics of FIGS. 2A-2B.

DETAILED DESCRIPTION

Leadless pacemaker designs described in the present disclosure provideimprovements over conventional pacemakers with leads and also over priorleadless pacemaker designs. The leadless pacemaker designs describedherein advantageously minimize biostimulator volume while increasingefficiency and cell life. Six design techniques described hereincontribute to reducing biostimulator volume.

First, the housing of the device's energy source can be used as part ofthe housing of the stimulator. This provides more compact constructionthan that of conventional pacemakers, which generally include a firstmetal housing containing the energy source, entirely enclosed within asecond metal housing containing the energy source housing, along withcircuitry.

Second, an energy source with high energy per unit volume and lowinternal resistance can be used within the leadless pacemaker. Bothfeatures decrease the amount of reactants necessary for a specifieddevice lifetime.

Additionally, the device's analog and digital functions can beimplemented with a single integrated circuit. This reduces board area,encapsulation volume, and interconnection area, thereby allowing all theinternal circuitry of the pacemaker to be contained within a smallerhousing and reducing overall biostimulator volume.

Fourth, the pacemaker can have a generally cylindrical form withdiameter not to exceed 7 mm, and preferably having a diameter that doesnot exceed 6 mm. In some embodiments, pacemakers utilizing the design ofthis disclosure can have dimensions of approximately 6 mm in diameterand approximately 3.5 cm in length, for a total volume of approximately1 cc and a mass of approximately 2 gm. This enables percutaneousdelivery of the biostimulator through the vasculature. To provide highenergy per unit volume and low internal resistance with this form,chemical cell manufacturers propose lithium carbon mono-fluoride (“CFx”)cells with “bobbin” construction, symmetric around the cell's long axis,with the lithium anode arranged along the cell housing's inside wall.Thus, in some embodiments the cell housing forms the cell's negativeterminal (“negative can”).

Another improvement includes providing efficient stimulation via a firstsmall-surface-area electrode (“tip”), and a second large-surface-areaelectrode (“ring” or “can”). The small tip provides a high electricfield gradient to induce stimulation. The large ring or can provides alow spreading resistance to minimize electrical losses. To preventcorrosion, arrhythmia induction, and elevated pacing thresholds,stimulators generally provide a pulse with the tip negative with respectto the can (“positive can”).

Finally, another improvement disclosed herein includes implementingmixed analog and digital functions on a single integrated circuit withminimal substrate area. In some embodiments, the integrated circuitsused in the leadless pacemakers described herein can include only p-typeprocesses where no point on the chip can have a voltage below thesubstrate voltage (“negative ground”).

FIG. 1 shows an external view of a leadless pacemaker or biostimulator100. The pacemaker 100 can comprise energy source or cell 101, pacingelectronics 102, tip electrode 103, insulator 104, and fixation feature105. Electronics 102 can include a single p-type substrate ASIC. Thepacemaker 100 can comprise an outer housing 107, which in thisembodiment is a combination of cell housing 106 (surrounding cell 101)and circuit housing 108 (surrounding electronics 102). The cell housing106 can act as an electrode (e.g. a ring electrode). In someembodiments, the housings can comprise a conductive material such astitanium, 316L stainless steel, or other similar materials. The fixationfeature 105 can comprise a fixation helix or other screw-like featureconfigured to affix the pacemaker to cardiac tissue.

In the embodiment of FIG. 1, the negative terminal of the energy source101 can be connected to the cell housing 106, and the positive terminalof the cell can be connected to electronics 102 within circuit housing108. By connecting the negative terminal of the energy source to thecell housing, the cell housing can then be used as a ring or canelectrode for the pacemaker. Since the cell housing 106 is connected tothe negative terminal of the energy source 101 so as to act as a canelectrode, the combination can be referred to collectively within thisdisclosure as the “negative can”, “can electrode”, or “ring electrode”.Utilizing the cell housing as the negative can allows the pacemaker 100to be designed without requiring an additional pacemaker housing and/orring electrode around the energy source and cell housing, which cansignificantly reduce the size and cost of the pacemaker.

Insulator 104 can be configured to electrically isolate tip electrode103 from the rest of the device, including from the electronics and thenegative can. The insulator 104 can include a ceramic to metalfeedthrough or a glass to metal feedthrough to connect the tip electrodeto electronics 102, as known in the art. The tip electrode 103 can be,for example, a raised or “button” shaped electrode disposed on a distaltip of the housing. The tip electrode can be other shapes, includingsquare, rectangular, circular, flat, pointed, or otherwise shaped asknown in the art. In additional embodiments, the electrode can beintegrated into the fixation feature 105.

When the pacemaker of FIG. 1 is activated, stimulation current can flowfrom the cell housing 106, at positive polarity during the stimulationpulse, to tip electrode 103, at negative polarity during the stimulationpulse. Consequently the cell housing 106 also serves as the positivering electrode during stimulation. Insulator 104 separates the cellhousing (acting as a ring or can electrode) from the tip electrode 103,both physically and electrically during use. In order for the pacemaker100 of FIG. 1 to function properly when implanted in a heart of apatient, the tip electrode 103 must be driven negative with respect tothe ring or can electrode (e.g., cell housing 106) even though thecell's negative terminal is connected directly to the ring or canelectrode.

Traditionally, n-type substrate technology was available to pacemakerand pacemaker designers, who could connect the positive terminal of thecell to the n-type substrate and to the ring electrode, allowing thenegative terminal of the cell to create a negative voltage that would becommuted to the tip electrode. However, it is presently difficult tofind n-type substrates for use in these applications, so the presentinvention advantageously allows the tip electrode to be driven negativewith respect to the ring electrode while using a p-type substrate.

FIGS. 2A-2B are simplified schematic diagrams of pacing and sensingcircuitry 200, according to one embodiment. The pacing and sensingcircuitry 200 can be all or a portion of the circuitry found inelectronics 102 of FIG. 1. Reference to can electrode 106 and tipelectrode 103 can also be referring to the electrodes of FIG. 1.

In the illustrated embodiment, the pacing and sensing circuitry 200 canbe a single p-type substrate ASIC. This circuitry allows the tipelectrode of a pacemaker to be driven negative with respect to the canelectrode when constrained to using a p-type substrate and a lithium CFxcell. FIG. 2A shows switches 201-206 in a first state, occurring betweenstimulation pulses, with switches 201-204 closed and switches 205-206opened. FIG. 2B shows switches 201-206 in a second state, occurringwhile delivering a stimulation pulse, with switches 201-204 opened andswitches 205-206 closed.

In the first state, energy source 101 (which can be the energy source101 from FIG. 1) charges cell tank capacitor 207 and pacing tankcapacitor 208, through switches 201-203.

In the second state, the energy source 101 is switched out of thecircuit and pacing tank capacitor 208 discharges through switches205-206 through body load 210 and output coupling capacitor 209, forcingthe tip electrode 103 to go negative with respect to the can electrode106. When the biostimulator 100 described above operates in the secondstate, stimulation current flows from the can electrode (positiveelectrode, also shown as cell housing 106 in FIG. 1) to the electrodetip (negative electrode, shown as tip electrode 103 in FIG. 1).

Returning to the first state, output coupling capacitor 209 dischargesthrough switches 202 and 204, and body load 210. This ensures chargebalance through the electrodes. Resistor 211 represents theon-resistance of switch 204, selected to limit this charge-balancingcurrent. The resistance of resistor 211 can be chosen based on severalfactors, including the stimulation frequency, load impedance, andeffective output capacitance.

Integrated circuit ground 212 consequently is the most negative voltagein the system. During the stimulation pulse (e.g., when the circuit isin the second state), the negative terminal of energy source 101 “fliesup” from ground to the stimulating voltage on the positive terminal ofpacing tank capacitor, and the positive terminal of energy source 101“flies up” even higher but is disconnected. Cell tank capacitor 207maintains a supply voltage for other circuits (not shown). Aftercompletion of the stimulation pulse, the cell “flies down” so that itsnegative terminal is reconnected to ground and its positive terminal isreconnected to the positive terminal of cell tank capacitor 207. This“flying cell” configuration permits the cell negative terminal—thenegative cell housing or can electrode—to serve as the positive ring orcan for stimulation.

Protection device or devices 214 limit voltage between the can electrode106 (which is the negative terminal of energy source 101) and the tipelectrode 103, to protect the circuit 200 during defibrillation orelectrosurgery. The circuit 200 may include a sensing amplifier as theprotection device 214 to detect intrinsic or evoked activity in thestimulated organ. The amplifier can detect potentials between tip 103and can 106 (housing of energy source 101), and all circuitry in theamplifier can operate above ground potential 212.

A capacitive or inductive voltage converter (not shown) may optionallyreplace switch 203 to provide efficient charging of pacing capacitor 208at voltages different from that of energy source 101, as is known in theart.

FIG. 3 shows a simplified schematic diagram 300 corresponding to each ofswitches 204 and 205 from FIGS. 2A-2B, which require a novelimplementation because of potential presence of defibrillation orelectrosurgery voltages on the tip electrode (such as tip electrode 103described above). Each switch has a high terminal 304, low terminal 305,control terminal 303, and driver voltage 309. The switch is designed topass no current in the presence of defibrillation or electrosurgeryvoltages on the high terminal 304 as limited by protection devices inthe circuit (such as protection device 214 above).

When control terminal 303 is low, resistor 306 holds switch 301 off andcontrol terminal 303 holds switch 302 off, even with full protectedvoltage on 304. Because switches 301 and 302 are connected in oppositeconfigurations, their body diodes do not conduct. When control terminal303 is driven to the driver voltage 309 (for example, the voltage at thepositive terminal of cell tank capacitor 207 from FIGS. 2A-2B), switch302 turns on, and switches 308 and 307 turn switch 301 on.

FIG. 4 shows a simplified schematic diagram 400 of switch 206 from FIGS.2A-2B, which requires novel implementation because of potential presenceof defibrillation or electrosurgery voltages on the can electrode (suchas housing 106 described above). The switch has a high terminal 404, lowterminal 405, control terminal 403, and driver voltage 409. The switchis designed to pass no current in the presence of defibrillation orelectrosurgery voltages on the high terminal 404 as limited byprotection device 214. When control terminal 403 is low (at the voltageof 405), level shifter 406 output is at the voltage of 405, which holdsswitches 401 and 402 off, even with full protected voltage on 404.Because switches 401 and 402 are connected in opposite configurations,their body diodes do not conduct. When control terminal 403 is driven tothe driver voltage 409 (in this case the voltage at the positiveterminal of energy source 101, which during stimulation is higher thanthe can electrode voltage), then switches 401 and 402 turn on.

Switches 201 and 203 of FIGS. 2A-2B may each be implemented with aP-channel MOSFET, and switch 202 may be implemented with an N-channelMOSFET, all in a conventional manner.

As for additional details pertinent to the present invention, materialsand manufacturing techniques may be employed as within the level ofthose with skill in the relevant art. The same may hold true withrespect to method-based aspects of the invention in terms of additionalacts commonly or logically employed. Also, it is contemplated that anyoptional feature of the inventive variations described may be set forthand claimed independently, or in combination with any one or more of thefeatures described herein. Likewise, reference to a singular item,includes the possibility that there are plural of the same itemspresent. More specifically, as used herein and in the appended claims,the singular forms “a,” “and,” “said,” and “the” include pluralreferents unless the context clearly dictates otherwise. It is furthernoted that the claims may be drafted to exclude any optional element. Assuch, this statement is intended to serve as antecedent basis for use ofsuch exclusive terminology as “solely,” “only” and the like inconnection with the recitation of claim elements, or use of a “negative”limitation. Unless defined otherwise herein, all technical andscientific terms used herein have the same meaning as commonlyunderstood by one of ordinary skill in the art to which this inventionbelongs. The breadth of the present invention is not to be limited bythe subject specification, but rather only by the plain meaning of theclaim terms employed.

What is claimed is:
 1. A leadless cardiac pacemaker, comprising: pacingelectronics; a tip electrode electrically coupled to the pacingelectronics; a cell housing; an energy source disposed in the cellhousing, a negative terminal of the energy source being electricallyconnected to the cell housing, wherein: during a first state: the pacingelectronics are powered by the energy source; the energy source chargesat least a first capacitor; and the energy source charges a secondcapacitor; and during a second state: the first capacitor isdisconnected from the energy source; the pacing electronics areelectrically disconnected from the energy source and the pacingelectronics are powered by the first capacitor; and the second capacitoris discharged into the cell housing, resulting in an electrical circuitwhere the cell housing becomes a positive terminal of the electricalcircuit with respect to a negative terminal of the electrical circuitformed at the tip electrode.
 2. The leadless cardiac pacemaker of claim1, further comprising sensing electronics disposed an electronicshousing, wherein the pacing and sensing electronics comprise at leastone p-type substrate.
 3. The leadless cardiac pacemaker of claim 2,wherein the energy source comprises a lithium carbon mono-fluoride cell.4. The leadless cardiac pacemaker of claim 1, wherein the electricalcircuit conducts electricity through cardiac tissue.
 5. The leadlesscardiac pacemaker of claim 1, further comprising at least one switchthat prevents passage of electricity between the pacing electronics andthe cell housing during the second state.
 6. The leadless cardiacpacemaker of claim 1, wherein the pacing electronics comprise a firstset of switches and a second set of switches; and wherein the pacingelectronics are configured to: electrically connect, during the firststate and through the first set of switches, to the energy source, theenergy source being electrically connected to the first capacitor usingthe first set of switches, and the energy source being electricallyconnected to the second capacitor using the second set of switches; andelectrically disconnect, during the second state, from the energy sourceby opening the first set of switches such that the pacing electronicsare powered by the first capacitor, wherein stimulation occurs bydischarging the second capacitor into the cell housing and opening thesecond set of switches such that the energy source is disconnected fromthe second capacitor.
 7. The leadless cardiac pacemaker of claim 1,wherein the pacing electronics comprise an integrated circuit ground,wherein the pacing electronics are configured to operate in the firststate between stimulation pulses and the second state during astimulation pulse, wherein: when the pacing electronics operate in thesecond state, a negative terminal of the energy source goes from groundto a stimulating voltage due to discharge of the second capacitor, and apositive terminal of the energy source is disconnected from the pacingelectronics; and when the pacing electronics operate in the first state,the negative terminal of the energy source is connected to theintegrated circuit ground and the positive terminal of the energy sourceis connected to a positive terminal of the first capacitor.
 8. Theleadless cardiac pacemaker of claim 1, wherein the pacing electronicscomprise a switch having a high terminal, a low terminal, a controlterminal, a driver voltage, a resistor, and a first and secondsub-switch connected to the switch in opposite configurations, whereinthe switch is configured to pass no current when defibrillation orelectrosurgery voltages are present on the high terminal, wherein whenthe control terminal is at a voltage of the low terminal, the resistorholds the first sub-switch off and the control terminal holds the secondsub-switch off, and when the control terminal is driven to the drivervoltage, the first and second sub-switches turn on.
 9. The leadlesscardiac pacemaker of claim 1, wherein the pacing electronics comprise aswitch having a high terminal, a low terminal, a control terminal, adriver voltage, a level shifter, and first and second sub-switchesconnected to the switch in opposite configurations, wherein the switchis configured to pass no current when defibrillation or electrosurgeryvoltages are present on the high terminal, wherein when control terminalis at a voltage of the low terminal, voltage output of the lever shifteris at the voltage of the low terminal so that the first and secondsub-switches are off, and when the control terminal is driven to thedriver voltage, the first and second sub-switches turn on.
 10. Aleadless cardiac pacemaker, comprising: a tip electrode; pacingelectronics disposed on a p-type substrate; and an energy sourcedisposed in a cell housing, wherein: during a first state, the energysource powers the pacing electronics and charges a first capacitor and asecond capacitor, and wherein the energy source, the first capacitor,and the second capacitor are electrically disconnected from the tipelectrode; and during a second state, the first capacitor iselectrically connected to the pacing electronics and powers the pacingelectronics, the first capacitor being electrically disconnected fromthe energy source and the tip electrode, while the second capacitor isdischarged into the cell housing, resulting in a circuit where the cellhousing becomes a positive terminal of the circuit with respect to anegative terminal formed at the tip electrode.
 11. The leadless cardiacpacemaker of claim 10, wherein the energy source comprises a lithiumcarbon mono-fluoride cell.
 12. The leadless cardiac pacemaker of claim10 wherein the cell housing is configured to act as a can electrode. 13.The leadless cardiac pacemaker of claim 12 wherein there is no separatering or can electrode disposed around the cell housing.